Good afternoon Rich and welcome back,
Thank you for reaching out to us.
I would love to help you with this topic.
I believe this issue is caused by the fact that your version of the Events Calendar Pro is outdated by a couple of versions.
The best solution to this is:
- Deactivate and delete the Events Calendar Pro
- Download the latest version here: https://theeventscalendar.com/my-account/downloads/
- Install it via the Add new plugin interface
- Activate the plugin
- Try the license key again
Let me know how that goes.
Best regards,
Geoff B.